The present invention relates generally to the field of electric screwdrivers and, more particularly, to an electric screwdriver, or tool, which is operated by an AC universal motor and which incorporates a phase-gating controller.
In phase-gating controllers, according to the prior art, each half wave of the AC voltage is used either as a full wave or as a phase-gated wave, in order to achieve operation which is as uniform as possible and has a continuous speed characteristic.
Known electric screwdrivers are equipped with this type of controller. The use of continuous rotational speeds when screws, used in the construction of buildings or the like, are being driven home lead to ejection forces on the screwdriver which have to be counteracted manually. It is extremely difficult to drive building screws home flush in a continuous fashion. The incorporation of a pre-selected value of torque to overcome this problem is not practical for fasteners in wood materials or on wood structures, since virtually every screwing operation requires a different torque.
It is also known to provide an electric motor operated on an AC system with a squirrel-cage rotor for driving a circular saw with a blanking controller which connects the AC voltage intermittently to the motor as shown in German Patent DE 41 30 532A1. In this known circuit arrangement, the aim is to reduce the power consumption under specific operating condutions, such as during idling, in order to maintain the rotational speed corresponding to full operation. On the other hand, it is also possible to provide a reduction in rotational speed and torque in order to reduce the power consumption in partial-load operation. Such a blanking controller does not come into consideration from the point of view of power reduction for electric tools used in manual operation because the centrifugal masses are too small for continuous operation. Furthermore, a squirrel-cage induction motor cannot output a useful torque up to the point of standstill.
It is recommended in the specialist literature in the case of the use of blanking control, which is also termed burst-firing control, in conjunction with electric motors to compensate the fluctuations in rotational speed or torque by an additional centrifugal mass. A reference covering this application may be found in DE-Z Der Elektromeister +Deutsches Elektrohandwerk, 1973, Vol. 48, Issue 18, pages 1269, 1270. According to this reference, a burst-firing controller is understood to be a circuit arrangement in which the complete voltage and frequency are continually periodically applied to the relevant apparatus, such as the electric motor, and the apparatus is respectively separated from the system between applications of complete voltage and frequency. It goes without saying that a pulsating torque which is undesired per se is obtained in accordance with the periodic starting and stopping operations in the case of an electric motor. Thus, with electric tools phase-gating control has been used exclusively to date in order to achieve a torque characteristic which is as uniform as possible in conjunction with the use of each half wave of the feeding AC voltage during operation.